1. Field of the Invention
The invention relates to a photoresist for the production of structures in the submicron range.
2. Description of Related Art
A high-resolution resist for the production of structures in the submicron range, which can be developed in aqueous alkaline manner, is known from EP-OS 0 388 484. This resist consists of a base polymer which can be developed, and a photoactive component (as well as any other conventional additives which might be necessary), where the base polymer has anhydride functions as the groups which can be developed, and therefore impart the solution. The base polymers which contain anhydride groups, which are DUV-transparent and are suitable both for single-layer and for two-layer technology, preferably contain 10 to 55 mole-% maleic acid anhydride, and are obtained by copolymerization or terpolymerization with allyl trimethyl silane and styrene, as well as with a further monomer, for example maleinimide, if applicable.
Photoresists with such base polymers, which have diazonaphthoquinone or diazoketone derivatives as the photoactive components, * require a dose of about 70 to 80 mJ/cm.sup.2 for a zero bias exposure. This relatively high dose can be disadvantageous, because in production lines, for example for highly integrated microelectronic components, a long period of occupation of the cost-intensive exposure devices, (steppers), is required at low throughput. In production, a sensitivity of &lt;20 mJ/cm.sup.2 is *) upon DUV exposure, for example at a wave length of 248 nm, therefore generally demanded. When using resists containing silicon, of the type stated, in two-layer technology, there is a further problem that after the transfer of structure in the oxygen plasma, submicron structures are obtained in the bottom resist, which have lost more than 10% of their line width as compared with the structures in the top resist. However, this cannot be tolerated for production.
A photostructuring method is known from EP-OS 0 395 917, which allows the photolithographic transfer of submicron structures in two-layer technology. With this method, the advantages of base polymers, i.e. resists, which contain anhydrid groups, are utilized, and at the same time, a pattern transfer true to size is made possible, i.e. the exact mask dimensions are reproduced in the bottom resist in two-layer technology. This is done by chemical expansion of the structures containing anhydride groups in the top resist, to an extent that precisely corresponds to the loss in dimensions which occurs during the structure transfer in the oxygen plasma. With this method, again, however, a relatively high dose is required for structuring the top resist, specifically a dose of about 90 mJ/cm.sup.2 (in the DUV range).
Furthermore, with the stated photostructuring method, the opportunity to produce structure dimensions which are smaller than pre-determined by the physical resolution limit of the lithography technology used was opened up for the first time, i.e. the space width in the photoresist structure is reduced in size to below the resolution limit. This is done by chemical expansion of the resist structures which contain anhydride groups, by a measure of several nanometers up to several micrometers, accompanied by a corresponding space narrowing. In this method of procedure, which is possible both with single-layer technology and with two-layer technology, resists with diazoketone derivatives as the photoactive component are used, but these resists do not satisfy the production demands with regard to sensitivity.
A resist composition which works in a positive or negative manner is known from U.S. Pat. No. 4,491,628, in which the problem of the insufficient sensitivity of conventional resists is approached with the concept of so-called chemical amplification. For this purpose, the resist composition has a polymer with groups unstable in acids, and a photoinitiator which releases an acid when exposed. In this connection, base polymers which possess alkali-soluble groups, for example phenolic OH groups (in a vinyl phenol polymer), which are blocked by groups which can be split off by acid, for example a tert. butyl or tert. butoxycarbonyl group, are used. The polymer is thus non-soluble in alkali, for the time being. As the photoinitiator (or photo acid), which forms a strong acid upon exposure with DUV light, electron beams or X-rays, so-called Crivello salts, i.e. onium salts, such as triphenyl sulfonium hexafluorophosphate, are used.
The great sensitivity of resist compositions of the type stated above is explained by the fact that during a temperature treatment ("post exposure bake") which follows the acid during exposure, a single proton which was generated from the photo acid during exposure, specifically by a photon, catalytically splits off a plurality of the groups which can be split off by acid, i.e releases a plurality of alkali-soluble groups. In contrast, a maximum of one single alkali-soluble group can be produced per proton in conventional resists with diazo compounds (as the photoactive component). A sensitivity which reaches 5 to 55 mJ/cm.sup.2 with UV exposure, i.e. is 10 .mu.C/cm.sup.2 with electron beam exposure, is stated for the resist compositions indicated; furthermore, they show high contrast. However, disadvantages of these systems are that a transfer of structures true to size is not possible, that no narrow spaces can be produced, i.e. spaces with a width less than the resolution limit, and that either the DUV transparency, or the etch resistance to substrate etch processes is relatively low.
A photoresist composition for DUV and excimer laser lithography is known from U.S. Pat. Nos. 4,837,124 and 4,912,018 which also functions according to the principle of chemical amplification. For this purpose, the resist composition has 1 to 50% of a latent photo acid and 50 to 99% of a polymer which forms a film and contains imide groups, both dissolved in a solvent. In this connection, a sufficient number of the imide groups are blocked with groups unstable in acid to make the polymer alkali-insoluble; such groups are oxycarbonyl groups, especially the tert. butoxycarbonyl group. The groups unstable in acid are split off by means of the photo acid, in the manner described, which causes the polymer to become alkali-soluble, due to the imide groups which occur as a result. The advantage of this composition is an improved resolution capacity, but it demonstrates the same disadvantages as the resist composition according to U.S. Pat. No. 4,491,628. In addition, it is disadvantageous that the introduction of the groups unstable in acid by means of a polymer-analogous reaction cannot be controlled, and does not proceed in reproducible manner, and therefore also does not take place in quantitative manner.